Multispectral biometric sensor

ABSTRACT

Methods and systems are provided for biometric sensing. An illumination subsystem provides light at discrete wavelengths to a skin site of an individual. A detection subsystem receives light scattered from the skin site. A computational unit is interfaced with the detection system. The computational unit has instructions for deriving a spatially distributed multispectral image from the received light at the discrete wavelengths. The computational unit also has instructions for comparing the derived multispectral image with a database of multispectral images to identify the individual.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/818,698, entitled “MULTISPECTRAL BIOMETRIC SENSOR,” filed Apr. 5,2004 by Robert K. Rowe et al., (“the parent application”), the entiredisclosure of which is incorporated herein by reference for allpurposes. The parent application is a nonprovisional of, and claims thebenefit of the filing date of, each of the following provisionalapplications, the entire disclosure of each of which is alsoincorporated herein by reference for all purposes: U.S. Prov. Pat. Appl.No. 60/460,247, entitled “NONINVASIVE ALCOHOL MONITOR,” filed Apr. 4,2003; U.S. Prov. Pat. Appl. No. 60/483,281, entitled “HYPERSPECTRALFINGERPRINT READER,” filed Jun. 27, 2003 by Robert K. Rowe et al.; U.S.Prov. Pat. Appl. No. 60/504,594, entitled “HYPERSPECTRALFINGERPRINTING,” filed Sep. 18, 2003; and U.S. Prov. Pat. Appl. No.60/552,662, entitled “OPTICAL SKIN SENSOR FOR BIOMETRICS,” filed Mar.10, 2004.

This application is also related to U.S. patent application Ser. No.09/874,740, entitled “APPARATUS AND METHOD OF BIOMETRIC DETERMINATIONUSING SPECIALIZED OPTICAL SPECTROSCOPY SYSTEM,” filed Jun. 5, 2001, theentire disclosures of both of which are incorporated herein by referencefor all purposes

BACKGROUND OF THE INVENTION

This application relates generally to biometrics. More specifically,this application relates to methods and systems for performing biometricmeasurements with a multispectral imaging sensor, and to methods andsystems for measuring in vivo levels of alcohol or other analytes.

“Biometrics” refers generally to the statistical analysis ofcharacteristics of living bodies. One category of biometrics includes“biometric identification,” which commonly operates under one of twomodes to provide automatic identification of people or to verifypurported identities of people. Biometric sensing technologies measurethe physical features or behavioral characteristics of a person andcompare those features to similar prerecorded measurements to determinewhether there is a match. Physical features that are commonly used forbiometric identification include faces, irises, hand geometry, veinstructure, and fingerprint patterns, which is the most prevalent of allbiometric-identification features. Current methods for analyzingcollected fingerprints include optical, capacitive, radio-frequency,thermal, ultrasonic, and several other less common techniques.

Most of the fingerprint-collection methods rely on measuringcharacteristics of the skin at or very near the surface of a finger. Inparticular, optical fingerprint readers typically rely on the presenceor absence of a difference in the index of refraction between the sensorplaten and the finger placed on it. When an air-filled valley of thefingerprint is above a particular location of the platen, total internalreflectance (“TIR”) occurs in the platen because of the air-platen indexdifference. Alternatively, if skin of the proper index of refraction isin optical contact with the platen, then the TIR at this location is“frustrated,” allowing light to traverse the platen-skin interface. Amap of the differences in TIR across the region where the finger istouching the platen forms the basis for a conventional opticalfingerprint reading. There are a number of optical arrangements used todetect this variation of the optical interface in both bright-field anddark-field optical arrangements. Commonly, a single, quasimonochromaticbeam of light is used to perform this TIR-based measurement.

There also exists non-TIR optical fingerprint sensors. In most cases,these sensors rely on some arrangement of quasimonochromatic light toilluminate the front, sides, or back of a fingertip, causing the lightto diffuse through the skin. The fingerprint image is formed due to thedifferences in light transmission across the skin-platen boundary forthe ridge and valleys. The difference in optical transmission are due tochanges in the Fresnel reflection characteristics due to the presence orabsence of any intermediate air gap in the valleys, as known to one offamiliarity in the art.

Optical fingerprint readers are particularly susceptible to imagequality problems due to non-ideal conditions. If the skin is overly dry,the index match with the platen will be compromised, resulting in poorimage contrast. Similarly, if the finger is very wet, the valleys mayfill with water, causing an optical coupling to occur all across thefingerprint region and greatly reducing image contrast. Similar effectsmay occur if the pressure of the finger on the platen is too little ortoo great, the skin or sensor is dirty, the skin is aged and/or worn, oroverly fine features are present such as may be the case for certainethnic groups and in very young children. These effects decrease imagequality and thereby decrease the overall performance of the fingerprintsensor. In some cases, commercial optical fingerprint readersincorporate a thin membrane of soft material such as silicone to helpmitigate these effects and restore performance. As a soft material, themembrane is subject to damage, wear, and contamination, limiting the useof the sensor without maintenance.

Biometric sensors, particularly fingerprint biometric sensors, aregenerally prone to being defeated by various forms of spoof samples. Inthe case of fingerprint readers, a variety of methods are known in theart for presenting readers with a fingerprint pattern of an authorizeduser that is embedded in some kind of inanimate material such as paper,gelatin, epoxy, latex, and the like. Thus, even if a fingerprint readercan be considered to reliably determine the presence or absence of amatching fingerprint pattern, it is also critical to the overall systemsecurity to ensure that the matching pattern is being acquired from agenuine, living finger, which may be difficult to ascertain with manycommon sensors.

Another way in which some biometric systems may be defeated is throughthe use of a replay attack. In this scenario, an intruder records thesignals coming from the sensor when an authorized user is using thesystem. At a later time, the intruder manipulates the sensor system suchthat the prerecorded authorized signals may be injected into the system,thereby bypassing the sensor itself and gaining access to the systemsecured by the biometric.

A common approach to making biometric sensors more robust, more secure,and less error-prone is to combine sources of biometric signals using anapproach sometimes referred to in the art as using “dual,”“combinatoric,” “layered,” “fused,” or “multifactor” biometric sensing.To provide enhanced security in this way, biometric technologies arecombined in such a way that different technologies measure the sameportion of the body at the same time and are resistant to being defeatedby using different samples or techniques to defeat the different sensorsthat are combined. When technologies are combined in a way that theyview the same part of the body they are referred to as being “tightlycoupled.”

The accuracy of noninvasive optical measurements of physiologicalanalytes such as glucose, alcohol, hemoglobin, urea, and cholesterol canbe adversely affected by variation of the skin tissue. In some cases itis advantageous to measure one or more physiological analytes inconjunction with a biometric measurement. Such dual measurement haspotential interest and application to both commercial andlaw-enforcement markets.

There is accordingly a general need in the art for improved methods andsystems for biometric sensing and analyte estimation using multispectralimaging systems and methods.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention thus provide methods and systems forbiometric sensing and physiological analyte estimation. The embodimentsof the present invention collect multispectral image data that representspatio-spectral information from multiple skin features at variousdepths and positions within an image volume. The information from thedifferent features can be advantageously combined to provide for methodsof biometric identification, including identity verification. As well,the multispectral image data may be processed to provide informationabout the authenticity or liveness state of a sample. The multispectralimage data may also be used to ascertain information about the presenceand amount of particular physiological analytes that may be present inthe tissue at the image location.

Embodiments of the invention provide methods and systems for assessingskin composition and structure in a certain location on the body usingoptical techniques. When light of a particular wavelength enters theskin, it is subject to optical interactions that include absorbance andscatter. Due to the optical scatter, a portion of the light willgenerally be diffusely reflected from the skin after entering the skinat the illumination point. An image of the light thus reflected containsinformation about the portion of the skin that the light passes throughwhile traveling from the point of illumination to detection. Differentwavelengths of light will interact with skin differently. Due to theproperties of certain skin components, certain wavelengths of light willinteract more or less strongly with certain components and structures.As well, certain wavelengths of light will travel greater distances intoand through the skin before being scattered back out of the skin anddetected. Accurate measurement of the spatial characteristics of lightthat is diffusely reflected from skin thus contains information aboutthe components and structures in the skin that interacted with light ofa certain wavelength. Similar measurements made using light of multipleand different illumination wavelengths provides additional informationabout the skin composition and structure.

In one set of embodiments, a sensor system is provided. An illuminationsubsystem is disposed to provide light at a plurality of discretewavelengths to a skin site of an individual. A detection subsystem isdisposed to receive light scattered from the skin site. A computationalunit is interfaced with the detection system. The computational unit hasinstructions for deriving a spatially distributed multispectral imagefrom the received light at the plurality of discrete wavelengths. Thecomputational unit also has instructions for comparing the derivedmultispectral image with a database of multispectral images to identifythe individual.

The identification of the individual may be performed differently indifferent embodiments. In one embodiment, the instructions for comparingthe derived multispectral image with the database comprise instructionsfor searching the database for an entry identifying a multispectralimage consistent with the derived multispectral image. In anotherembodiment, the instructions for comparing the derived multispectralimage with the database comprise instructions for comparing the derivedmultispectral image with the multispectral image at an entry of thedatabase corresponding to a purported identity of the individual toverify the purported identity.

The illumination subsystem may comprise a light source that provides thelight to the plurality of discrete wavelengths, and illumination opticsto direct the light to the skin site. In some instances, a scannermechanism may also be provided to scan the light in a specified pattern.The light source may comprise a plurality of quasimonochromatic lightsources, such as LEDs or laser diodes. Alternatively, the light sourcemay comprise a broadband light source, such as an incandescent bulb orglowbar, and a filter disposed to filter light emitted from the broadband source. The filter may comprise a continuously variable filter inone embodiment. In some cases, the detection system may comprise a lightdetector, an optically dispersive element, and detection optics. Theoptically dispersive element is disposed to separate wavelengthcomponents of the received light, and the detection optics direct thereceived light to the light detector. In one embodiment, both theillumination and detection subsystems comprise a polarizer. Thepolarizers may be circular polarizers, linear polarizers, or acombination. In the case of linear polarizers, the polarizers may besubstantially crossed relative to each other.

The sensor system may comprise a platen to contact the skin site, or thesensor system may be configured for noncontact operation. The platen maybe adapted for the skin site to be swiped over a surface of the platen.In one such embodiment, the platen comprises an optically clear rollerthat the finger can roll across with a swipe motion. In such anembodiment, the instructions for deriving the spatially distributedmultispectral image include instructions for building up themultispectral image from light received from different portions of theskin site as the skin site is rolled.

The illumination subsystem may comprise a plurality of illuminationsubsystems. In different embodiments, the plurality of discretewavelengths are provided sequentially or are provided substantiallysimultaneously and with an identifiable encoding. Suitable wavelengthsfor the plurality of discrete wavelengths include wavelengths betweenabout 400 nm and 2.5 μm.

In some embodiments, the sensor system may have additional components toallow the estimation of other parameters. For instance, in oneembodiment, the computational system further has instructions forderiving spectral-distribution characteristics from the received light.Such spectral-distribution characteristics may be used to determine ananalyte concentration in tissue below a surface of the skin site, suchas a concentration of alcohol, glucose, hemoglobin, urea, andcholesterol. In another embodiment, the computational system further hasinstructions for determining a liveness state from the derivedspectral-distribution characteristics.

In a second set of embodiments, methods are provided for identifying anindividual. A skin site of the individual is illuminated at a pluralityof discrete wavelengths. Light scattered from the skin site is received.A spatially distributed multispectral image is derived from the receivedlight at the plurality of discrete wavelengths. The derivedmultispectral image data or one or more of its parts are compared with adatabase of derived multispectral images. Various of the embodimentsinclude aspects discussed above in connection with embodiments for thesensor system. In some instances, the methods allow generation ofmeasurement sequences that are not constant for all samples. In oneembodiment, a sequence of illumination wavelengths is changed betweenmeasurements. In another embodiment, the selection of which illuminationwavelengths are used to illuminate the skin are changed betweenmeasurements.

In a third set of embodiments, a sensor system is provided. Anillumination subsystem is disposed to provide light at a plurality ofdiscrete wavelengths to a sample. A detection subsystem is disposed toreceive light scattered within tissue of the sample. A computationalunit is interfaced with the detection subsystem. The computational unithas instructions for deriving multispectral characteristics of thereceived light at the plurality of distinct wavelengths. Thecomputational unit also has instructions for determining a livenessstate of the tissue from the derived multispectral characteristics. Inone such embodiment, the liveness state is determined by pixelatingspatial distributions of the derived multispectral characteristics. Anmultivariate factor analysis is performed on a matrix having entries ina first dimension corresponding to a pixel of a pixelated spatialdistribution and having entries in a second dimension corresponding toone of the plurality of distinct wavelengths. In addition, various ofthe embodiments may include aspects discussed above in connectionembodiments for other sensor systems.

In a fourth set of embodiments, a method is provided for determining aliveness state of a sample. The sample is illuminated with light at aplurality of discrete wavelengths. Light scattered within tissue of thesample is received. Multispectral characteristics of the received lightare derived at the plurality of discrete wavelengths. A liveness stateof the tissue is determined from the derived multispectralcharacteristics to ensure that the derived characteristics of the sampleare consistent with the characteristics anticipated from an authenticsample. Various of the embodiments may include aspects discussed abovefor other sets of embodiments.

In a fifth set of embodiments, a method is provided for determining ablood-alcohol level of an individual. Electromagnetic radiationemanating from tissue of the individual in response to propagation ofelectromagnetic radiation into the tissue of the individual is received.Spectral properties of the received electromagnetic radiation areanalyzed. The blood-alcohol level is determined from the analyzedspectral properties.

The spectral properties may be analyzed over specific wavelength rangesin specific embodiments. For example, in one embodiment amplitudes ofthe received electromagnetic radiation are determined within awavelength range of 2.1-2.5 μm. This range includes the specificwavelengths of 2.23 μm, 2.26 μm, 2.28 μm, 2.30 μm, 2.32 μm, 2.25 μm, and2.38 μm, at one or more of which amplitudes may be determined in aspecific embodiment. In another embodiment, amplitudes of the receivedelectromagnetic radiation are determined within a wavelength range of1.5-1.9 μm. This range includes 1.67 μm, 1.69 μm, 1.71 μm, 1.73 μm, 1.74μm 1.76 μm and 1.78 μm, at one or more of which amplitudes may bedetermined in a specific embodiment.

In a sixth set of embodiments, an apparatus is provided for determininga blood-alcohol level of an individual. A receiver is adapted to receiveelectromagnetic radiation emanating from tissue of the individual inresponse to propagation of electromagnetic radiation into the tissue ofthe individual. A computer readable-storage medium is coupled with aprocess and has a computer-readable program embodied therein fordirecting operation of the processor. The computer-readable programincludes instructions for analyzing spectral properties of the receivedelectromagnetic radiation and instructions for determining theblood-alcohol level from the analyzed spectral properties.

In some embodiments, the methods and/or apparatus of the invention maybe embodied in devices, such as motor vehicles, whose access and/oroperation may be dependent on the determination of the blood-alcohollevel. Furthermore, the use of alcohol monitoring may be coupled withbiometric identifications in some embodiments. For example, accessand/or operation of devices embodying combined alcohol-monitoring andbiometric-identification devices may be dependent on a combination ofalcohol-monitoring and biometric-identification determinations. In oneembodiment, the biometric identification is performed with the samemultispectral data used to perform the alcohol estimation.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings wherein like reference labels are usedthroughout the several drawings to refer to similar components. In someinstances, reference labels include a numerical portion followed by alatin-letter suffix; reference to only the numerical portion ofreference labels is intended to refer collectively to all referencelabels that have that numerical portion but different latin-lettersuffices.

FIG. 1 provides a front view of a multispectral biometric sensor in oneembodiment of the invention;

FIG. 2A provides a side view of a multispectral biometric sensor shownin one embodiment;

FIG. 2B provides a side view of a multispectral biometric sensor shownin another embodiment;

FIG. 3 provides a front view of a computer tomographic imagingspectrometer (“CTIS”) in one embodiment of the invention;

FIG. 4 provides a top view of a swipe sensor in an embodiment of theinvention;

FIG. 5 illustrates a multispectral datacube generated in accordance withembodiments of the invention;

FIG. 6 is a graphical illustration of the effects of skin scatter;

FIG. 7 provides a graphical illustration of the effects of bloodabsorbance;

FIG. 8 provides examples of different illumination characteristics thatmay be used in embodiments of the invention;

FIG. 9A provides a flow diagram illustrating a method for using analcohol monitor in accordance with an embodiment of the invention;

FIG. 9B provides a flow diagram illustrating a method for using acombination of an alcohol monitor and a biometric sensor with anembodiment of the invention;

FIG. 9C provides a flow diagram illustrating a method for accommodatingoptical drift in embodiments of the invention; and

FIG. 10 provides a schematic representation of a computer system thatmay be used to manage functionality of alcohol monitors in accordancewith embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION 1. Overview

Embodiments of the invention provide methods and systems that allow forthe collection and processing of integrated, multifactor biometricmeasurements. These integrated, multifactor biometric measurements mayprovide strong assurance of a person's identity, as well as of theauthenticity of the biometric sample being taken. In some embodiments, asensor provides a plurality of discrete optical wavelengths thatpenetrate the surface of the skin, and scatter within the skin and/orunderlying tissue. As used herein, reference to “discrete wavelengths”is intended to refer to sets of wavelengths or wavelength bands that aretreated as single binned units—for each binned unit, information isextracted only from the binned unit as a whole, and not from individualwavelength subsets of the binned unit. In some cases, the binned unitsmay be discontinuous so that when a plurality of discrete wavelengthsare provided, some wavelength between any pair of the wavelengths orwavelength bands is not provided, but this is not required in allembodiments. In one embodiment, the optical wavelengths are within theultraviolet—visible—near-infrared wavelength range. A portion of thelight scattered by the skin and/or underlying tissue exits the skin andis used to form a multispectral image of the structure of the tissue atand below the surface of the skin. As used herein, the term“multispectral” is intended to be construed broadly as referring tomethods and systems that use multiple wavelengths, and thus includesimaging systems that are “hyperspectral” or “ultraspectral” as thoseterms are understood by those of skill in the art. Because of thewavelength-dependent properties of the skin, the image formed from eachwavelength of light is usually different from images formed at otherwavelengths. Accordingly, embodiments of the invention collect imagesfrom each of the wavelengths of light in such a way that characteristicspectral and spatial information may be extracted by an algorithmapplied to the resulting multispectral image data.

In some applications, it may be desirable to estimate other parametersand characteristics of a body, either independently or in combinationwith a biometric measurement. For example, in one specific suchembodiment, an ability is provided to measure blood-alcohol levels of aperson simultaneously with measurement of a fingerprint pattern; such anembodiment has applications to law enforcement as well as to a varietyof commercial applications including restricting motor-vehicle access.In this way, the analyte measurement and the identity of the person onwhom the measurement is made may be inextricably linked.

Skin composition and structure is very distinct, very complex, andvaries from person to person. By performing optical measurements of thespatio-spectral properties of skin and underlying tissue, a number ofassessments may be made. For example, a biometric-identificationfunction may be performed to identify or verify whose skin is beingmeasured, a liveness function may be performed to assure that the samplebeing measured is live and viable skin and not another type of material,estimates may be made of a variety of physiological parameters such asage gender, ethnicity, and other demographic and anthropometriccharacteristics, and/or measurements may be made of the concentrationsof various analytes and parameters including alcohol, glucose, degreesof blood perfusion and oxygenation, biliruben, cholesterol, urea, andthe like.

The complex structure of skin may be used in different embodiments totailor aspects of the methods and systems for particular functions. Theoutermost layer of skin, the epidermis, is supported by the underlyingdermis and hypodermis. The epidermis itself may have five identifiedsublayers that include the stratum comeum, the stratum lucidum, thestratum granulosum, the stratum spinosum, and the stratum germinativum.Thus, for example, the skin below the top-most stratum comeum has somecharacteristics that relate to the surface topography, as well as somecharacteristics that change with depth into the skin. While the bloodsupply to skin exists in the dermal layer, the dermis has protrusionsinto the epidermis known as “dermal papillae,” which bring the bloodsupply close to the surface via capillaries. In the volar surfaces ofthe fingers, this capillary structure follows the structure of thefriction ridges on the surface. In other locations on the body, thestructure of the capillary bed may be less ordered, but is stillcharacteristic of the particular location and person. As well, thetopography of the interface between the different layers of skin isquite complex and characteristic of the skin location and the person.While these sources of subsurface structure of skin and underlyingtissue represent a significant noise source for non-imaging opticalmeasurements of skin for biometric determinations or analytemeasurements, the structural differences are manifested by spectralfeatures compared through embodiments of the invention.

In some instances, inks, dyes and/or other pigmentation may be presentin portions of the skin as topical coating or subsurface tattoos. Theseforms of artificial pigmentation may or may not be visible to the nakedhuman eye. However, if one or more wavelengths used by the apparatus ofthe present invention is sensitive to the pigment, the sensor can beused in some embodiments to verify the presence, quantity and/or shapeof the pigment in addition to other desired measurement tasks.

In general, embodiments of the present invention relate to methods andsystems for collecting spatio-spectral information in the form ofmultispectral images or datacubes. In certain instances, the desiredinformation is contained in just a portion of the entire multispectraldatacube. For example, estimation of a uniformly distributed, spectrallyactive compound may require just the measure spectral characteristics,which can be extracted from the overall multispectral datacube. In suchcases, the overall system design may be simplified to reduce oreliminate the spatial component of the collected data by reducing thenumber of image pixels, even to a limit of a single pixel. Thus, whilethe systems and methods disclosed are generally described in the contextof multispectral imaging, it will be recognized that the inventionencompasses similar measurements in which the degree of imaging isgreatly reduced, even to the point where there is a single detectorelement.

2. Exemplary Embodiments

One embodiment of the invention is depicted with the schematic diagramof FIG. 1, which shows a front view of a multispectral biometric sensor101. The multispectral sensor 101 comprises an illumination subsystem121 having one or more light sources 103 and a detection subsystem 123with an imager 115. The figure depicts an embodiment in which theillumination subsystem 121 comprises a plurality of illuminationsubsystems 121 a and 121 b, but the invention is not limited by thenumber of illumination or detection subsystems 121 or 123. For example,the number of illumination subsystems 121 may conveniently be selectedto achieve certain levels of illumination, to meet packagingrequirements, and to meet other structural constraints of themultispectral biometric sensor 101. Illumination light passes from thesource 103 through illumination optics 105 that shape the illuminationto a desired form, such as in the form of flood light, light lines,light points, and the like. The illumination optics 105 are shown forconvenience as consisting of a lens but may more generally include anycombination of one or more lenses, one or more mirrors, and/or otheroptical elements. The illumination optics 105 may also comprise ascanner mechanism (not shown) to scan the illumination light in aspecified one-dimensional or two-dimensional pattern. The light source103 may comprise a point source, a line source, an area source, or maycomprise a series of such sources in different embodiments. In oneembodiment, the illumination light is provided as polarized light, suchas by disposing a linear polarizer 107 through which the light passesbefore striking a finger 119 or other skin site of the person beingstudied.

In some instances, the light source 103 may comprise one or morequasimonochromatic sources in which the light is provided over a narrowwavelength band. Such quasimonochromatic sources may include suchdevices as light-emitting diodes, laser diodes, or quantum-dot lasers.Alternatively, the light source 103 may comprise a broadband source suchas in incandescent bulb or glow bar. In the case of a broadband source,the illumination light may pass through a bandpass filter 109 to narrowthe spectral width of the illumination light. In one embodiment, thebandpass filter 109 comprises one or more discrete optical bandpassfilters. In another embodiment, the bandpass filter 109 comprises acontinuously variable filter that moves rotationally or linearly (orwith a combination of rotational and linear movement) to change thewavelength of illumination light. In still another embodiment, thebandpass filter 109 comprises a tunable filter element such as aliquid-crystal tunable filter, an acousto-optical tunable filter, atunable Fabry-Perot filter or other filter mechanism known to oneknowledgeable in the art.

After the light from the light source 103 passes through theillumination optics 105, and optionally the optical filter 109 and/orpolarizer 107, it passes through a platen 117 and illuminates the finger119 or other skin site. The sensor layout and components mayadvantageously be selected to minimize the direct reflection of theillumination into the detection optics 113. In one embodiment, suchdirect reflections are reduced by relatively orienting the illuminationsubsystem 121 and detection subsystem 123 such that the amount ofdirectly reflected light detected is minimized. For instance, opticalaxes of the illumination subsystem 121 and the detection subsystem 123may be placed at angles such that a mirror placed on the platen 117 doesnot direct an appreciable amount of illumination light into thedetection subsystem 123. In addition, the optical axes of theillumination and detection subsystems 121 and 123 may be placed atangles relative to the platen 117 such that the angular acceptance ofboth subsystems is less than the critical angle of the system; such aconfiguration avoids appreciable effects due to total internalreflectance between the platen 117 and the skin site 119.

An alternative mechanism for reducing the directly reflected light makesuse of optical polarizers. Both linear and circular polarizers can beemployed advantageously to make the optical measurement more sensitiveto certain skin depths, as known to one familiar in the art. In theembodiment illustrated in FIG. 1, the illumination light is polarized bylinear polarizer 107. The detection subsystem 123 may then also includea linear polarizer 111 that is arranged with its optical axissubstantially orthogonal to the illumination polarizer 107. In this way,light from the sample must undergo multiple scattering events tosignificantly change its state of polarization. Such events occur whenthe light penetrates the surface of the skin and is scattered back tothe detection subsystem 123 after many scatter events. In this way,surface reflections at the interface between the platen 117 and the skinsite 119 are reduced.

The detection subsystem 123 may incorporate detection optics thatcomprise lenses, mirrors, and/or other optical elements that form animage of the region near the platen surface 117 onto the detector 115.The detection optics 113 may also comprise a scanning mechanism (notshown) to relay portions of the platen region onto the detector 115 insequence. In all cases, the detection subsystem 123 is configured to besensitive to light that has penetrated the surface of the skin andundergone optical scattering within the skin and/or underlying tissuebefore exiting the skin.

The illumination subsystem 121 and detection subsystem 123 may beconfigured to operate in a variety of optical regimes and at a varietyof wavelengths. One embodiment uses light sources 103 that emit lightsubstantially in the region of 400-1000 nm; in this case, the detector115 may be based on silicon detector elements or other detector materialknown to those of skill in the art as sensitive to light at suchwavelengths. In another embodiment, the light sources 103 may emitradiation at wavelengths that include the near-infrared regime of1.0-2.5 μm, in which case the detector 115 may comprise elements madefrom InGaAs, InSb, PbS, MCT, and other materials known to those of skillin the art as sensitive to light at such wavelengths.

A side view of one of the embodiments of the invention is shown with theschematic drawing provided in FIG. 2A. For clarity, this view does notshow the detection subsystem, but does show an illumination subsystem121 explicitly. The illumination subsystem 121 in this embodimentincludes two discrete light sources 203 and 205 that have differentwavelength characteristics. For example, the light sources 203 and 205may be quasimonochromatic sources such as LEDs, which do not require anoptical filter. Sources 203 a, 203 b, and 203 c may provide illuminationwith substantially the same first wavelength while sources 205 a, 205 b,and 205 c may provide illumination with substantially the same secondwavelength, different from the first wavelength. As shown, theillumination optics in FIG. 2A are configured to provide floodillumination, but in alternative embodiments could be arranged toprovide line, point, or other patterned illumination by incorporation ofcylindrical optics, focusing optics, or other optical components asknown to those knowledgeable in the art.

An exemplary measurement sequence for the system shown in FIG. 2Acomprising activating the first light sources 203 and collecting aresulting image. After the image is acquired, the first light sources203 are turned off and the second light sources 205 are activated at adifferent wavelength, and a resulting image is collected. For a sensorhaving more than one wavelength of light source, thisillumination-measurement sequence is repeated for all the differentwavelengths used in the sensor. It will also be appreciated thatsubstantially the same sequence may be used in embodiments in which thewavelength characteristics of light are determined by states of tunableoptical filters, variable optical filters, moveable discrete opticalfilters, and the like. Also, an alternative mechanism for collectingimages at multiple wavelengths may incorporate an encoding method toidentify light of each wavelength when multiple wavelengths areilluminated at a given time. The data from the entire illuminationsequence is then collected in such a way that the individual wavelengthresponses are determined from the encoding using methods known to thoseof skill in the art. Illumination techniques thus include round-robin,frequency-division modulation, Hadamard encoding, and others.

The sequence of illumination of the light sources may be changed frommeasurement to measurement. This variability may be introduced to thwartreplay attacks where a set of valid signals is recorded and replayed ata later time to defeat the biometric sensor. The measurement variabilityfrom sample to sample may also extend in some embodiments to using onlya subset of available illumination wavelengths, which are then comparedwith the corresponding subset of data in an enrollment dataset.

The array of light sources 203 and 205 need not actually be planar asshown in FIG. 2A. For example, in other embodiments, optical fibers,fiber bundles, or fiber optical faceplates or tapers could convey thelight from the light sources at some convenient locations to anillumination plane, where light is reimaged onto the finger. The lightsources could be controlled by turning the drive currents on and off asLEDs might be. Alternatively, if an incandescent source is used, rapidswitching of the light may be accomplished using some form of spatiallight modulator such as a liquid crystal modulator or usingmicroelectromechanical-systems (“MEMS”) technology to control apertures,mirrors, or other such optical elements.

The use of optical components such as optical fibers and fiber bundlesmay allow the structure of the multispectral biometric sensor to besimplified. One embodiment is illustrated in FIG. 2B, which shows theuse of optical fibers and electronic scanning of illumination sourcessuch as LEDs. Individual fibers 216 a connect each of the LEDs locatedat an illumination array 210 to an imaging surface, and other fibers 216b relay the reflected light back to the imaging device 212, which maycomprise a photodiode array or CCD array. The set of fibers 216 a and216 b thus defines an optical fiber bundle 214 used in relaying light.

Another embodiment of the invention is shown schematically with thefront view of FIG. 3. In this embodiment, the multispectral biometricsensor 301 comprises a broadband illumination subsystem 323 and adetection subsystem 325. As for the embodiment described in connectionwith FIG. 1, there may be multiple illumination subsystems 323 in someembodiments, with FIG. 3 showing a specific embodiment having twoillumination subsystems 323. A light source 303 comprised by theillumination subsystem 323 is a broadband illumination source such as anincandescent bulb or a glowbar, or may be any other broadbandillumination source known to those of skill in the art. Light from thelight source 303 passes through illumination optics 305 and a linearpolarizer 307, and may optionally pass through a bandpass filter 309used to limit the wavelengths of light over a certain region. The lightpasses through a platen 117 and into a skin site 119. A portion of thelight is diffusely reflected from the skin 119 into the detectionsubsystem 325, which comprises imaging optics 315 and 319, a crossedlinear polarizer 311, and a dispersive optical element 313. Thedispersive element 313 may comprise a one- or two-dimensional grating,which may be transmissive or reflective, a prism, or any other opticalcomponent known in the art to cause a deviation of the path of light asa function of the light's wavelength. In the illustrated embodiment, thefirst imaging optics 319 acts to collimate light reflected from the skin119 for transmission through the crossed linear polarizer 311 anddispersive element 313. Spectral components of the light are angularlyseparated by the dispersive element 313 and are separately focused bythe second imaging optics 315 onto a detector 317. As discussed inconnection with FIG. 1, the polarizers 307 and 311 respectivelycomprised by the illumination and detection subsystems 323 and 325 actto reduce the detection of directly reflected light at the detector 317.

The multispectral image generated from light received at the detector isthus a “coded” image in the manner of a computer tomographic imagingspectrometer (“CTIS”). Both wavelength and spatial information aresimultaneously present in the resulting image. The individual spectralpatterns may be obtained by mathematical inversion or “reconstruction”of the coded image.

The embodiments described above in connection with FIGS. 1-3 areexamples of “area” sensor configurations. In addition to such areasensor configurations, multispectral imaging sensors may be configuredas “swipe” sensors in some embodiments. One example of a swipe sensor isshown in top view with the schematic illustration of FIG. 4. In thisfigure, the illumination region 403 and detection region 405 of a sensor401 are substantially collinear. In some embodiments of a swipe sensor401, there may be more than a single illumination region. For example,there may be a plurality of illumination regions arranged on either sideof the detection region 405. In some embodiments, the illuminationregion 403 may partially or fully overlap the detection region 405. Themultispectral image data are collected with the sensor 401 by swiping afinger or other body part across the optically active region, asindicated by the arrow in FIG. 4. The corresponding linear sensor may bea stationary system or a roller system that may further include anencoder to record the position information and aid in stitching a fulltwo-dimensional image from a resulting series of image slices as knownto one knowledgeable in the art. When the roller system is used, afingertip or other skin site may be rolled over a roller that istransparent to the wavelengths of light used. The light is thensequentially received from discrete portions of the skin site, with themultispectral image being built up from light received from thedifferent portions.

The polarizers included with some embodiments may also be used to createor further accentuate the surface features. For instance, if theillumination light is polarized in a direction parallel (“P”) with thesampling platen and the detection subsystem incorporates a polarizer ina perpendicular orientation (“S”), then the reflected light is blockedby as much as the extinction ratio of the polarizer pair. However, lightthat crosses into the fingertip at a ridge point is optically scattered,which effectively randomizes the polarization. This allows a portion, onthe order of 50%, of the absorbed and re-emitted light to be observed bythe S-polarized imaging system.

The systems described in connection with the specific embodiments aboveare illustrative and are not intended to be limiting. There are numerousvariations and alternatives to the exemplary embodiments described abovethat are also within the intended scope of the invention. In manyinstances, the layout or order of the optical components may be changedwithout substantially affecting functional aspects of the invention. Forexample, in embodiments that use broadband illumination sources and oneor more optical filters, the filter(s) may be located at any of avariety of points in both the illumination and detection subsystems.Also, while the figures show the finger or other skin site from whichmeasurements are made being in contact with the platen, it will beevident that substantially the same measurements may be made withoutsuch contact. In such instances, the optical systems for illuminationand detection may be configured to illuminate and image the skin site ata distance. Some examples of such systems are provided in U.S. Prov.Pat. Appl. No. 60/552,662 entitled “OPTICAL SKIN SENSOR FOR BIOMETRICS,”filed Mar. 10, 2004, which has been incorporated by reference.

The embodiments described above produce a set of images of the skin siteat different wavelengths or produce data from which such a set may beproduced using reconstruction techniques, such as in the particular caseof the CTIS or encoded illumination subsystems. For purposes ofillustration, the following discussion is made with reference to such aset of spectral images, although it in not necessary to produce them forsubsequent biometric processing in those embodiments that do notgenerate them directly. An illustrative set of multispectral images isshown in FIG. 5, with the set defining a multispectral datacube 501.

One way to decompose the datacube 501 is into images that correspond toeach of the wavelengths used in illuminating the sample in themeasurement process. In the figure, five separate images 503, 505, 507,509, and 511 are shown, corresponding to five discrete illuminationwavelengths and/or illumination conditions (e.g. illumination pointsource at position X, Y). In an embodiment where visible light is used,the images might correspond, for example, to images generated usinglight at 450 nm, 500 nm, 550 nm, 600 nm, and 650 nm. Each imagerepresents the optical effects of light of a particular wavelengthinteracting with skin and, in the case of embodiments where the skin isin contact with a platen during measurement, represents the combinedoptical effects of light of a particular wavelength interacting withskin and also passing through the skin-platen interface. Due to theoptical properties of skin and skin components that vary by wavelength,each of the multispectral images 503, 505, 507, 509, and 511 will be, ingeneral, different from the others

The datacube may thus be expressed as R(X_(S), Y_(S), X_(I), Y_(I), λ)and describes the amount of diffusely reflected light of wavelength λseen at each image point X_(I), Y_(I) when illuminated at a source pointX_(S), Y_(S). Different illumination configurations (flood, line, etc.)can be summarized by summing the point response over appropriate sourcepoint locations. A conventional non-TIR fingerprint image F(X_(I),Y_(I)) can loosely be described as the multispectral data cube for agiven wavelength, λ_(o), and summed over all source positions:

${F\left( {X_{I},Y_{I}} \right)} = {\sum\limits_{Y_{S}}{\sum\limits_{X_{S}}{{R\left( {X_{S},Y_{S},X_{I},Y_{I},\lambda_{0}} \right)}.}}}$Conversely, the spectral biometric dataset S(λ) relates the measuredlight intensity for a given wavelength λ to the difference {right arrowover (D)} between the illumination and detection locations:S({right arrow over (D)},λ)=R(X _(I) −X _(S) ,Y _(I) −Y _(S),λ).The multispectral datacube R is thus related to both conventionalfingerprint images and to spectral biometric datasets. The multispectraldatacube R is a superset of either of the other two data sets andcontains correlations and other information that may be lost in eitherof the two separate modalities.

The optical interactions at the skin-platen interface will besubstantially the same at all wavelengths since the optical qualities ofthe platen material and the skin are not generally significantlydifferent over the range of wavelengths used and the optical interfacedoes not change substantially during the measurement interval. Lightmigrated from the skin to the platen, as well as from the platen to theskin, will be affected by Fresnel reflections at the optical interfaces.Thus, light that traverses an air gap will be less intense in thereceiving medium than light that does not cross an air gap. Thisphenomenon forms just one portion of the image information that iscontained in the multispectral datacube.

The light that passes into the skin and/or underlying tissue isgenerally affected by different optical properties of the skin and/orunderlying tissue at different wavelengths. Two optical effects in theskin and/or underlying tissue that are affected differently at differentwavelengths are scatter and absorbance. Optical scatter in skin tissueis generally a smooth and relatively slowly varying function ofwavelength, as shown in FIG. 6. Conversely, absorbance in skin isgenerally a strong function of wavelength due to particular absorbancefeatures of certain components present in the skin. For example, bloodhas certain characteristic absorbance features as shown in FIG. 7. Inaddition to blood, other substances that have significant absorbanceproperties in the spectral region from 400 nm to 2.5 μm and that arefound in skin and/or underlying tissue include melanin, water, carotene,biliruben, ethanol, and glucose.

The combined effect of optical absorbance and scatter causes differentillumination wavelengths to penetrate the skin to different depths. Thiseffect is illustrated schematically in FIG. 8, which depicts the opticalscattering that occurs in tissue for three different illumination pointson the surface of skin at three different wavelengths, shown with thesame scale. This phenomenon effectively causes the different spectralimages to have different and complementary information corresponding todifferent volumes of the illuminated tissue. In particular, thecapillary layers close to the surface of the skin have distinct spatialcharacteristics that can be imaged using wavelengths of light in whichblood is strongly absorbing.

Thus, the multispectral image datacube contains spatio-spectralinformation from multiple sources. Merely by way of example, for thecase of a measurement taken on the fingertip in contact with a platen,the resulting datacube contains effects due to: (i) the opticalinterface between the fingertip and the platen, similar to informationcontained in a conventional non-TIR fingerprint; (ii) the overallspectral characteristics of the tissue, which are distinct from personto person; (iii) the blood vessels close to the surface of the skin,similar to vein imaging; and (iv) the blood vessels and other spectrallyactive structures distributed deeper in the tissue. As such, embodimentsof the invention provide a mechanism for extracting biometric data frommultiple sources within the fingertip or other skin site being measured,thereby providing multifactor biometric-sensing applications.

Because of the complex wavelength-dependent properties of skin andunderlying tissue, the set of spectral values corresponding to a givenimage location has spectral characteristics that are well-defined anddistinct. These spectral characteristics may be used to classify themultispectral image data on a pixel-by-pixel basis. This assessment maybe performed by generating typical tissue spectral qualities from a setof qualified images. For example, the multispectral data shown in FIG. 5may be reordered as an N×5 matrix, where N is the number of image pixelsthat contain data from living tissue, rather than from a surroundingregion of air. An eigenanalysis or other factor analysis performed onthis set matrix produces the representative spectral features of thesetissue pixels. The spectra of pixels in a later data set may then becompared to such previously established spectral features using metricssuch as Mahalanobis distance and spectral residuals. If more than asmall number of image pixels have spectral qualities that areinconsistent with living tissue, then the sample is deemed to benon-genuine and rejected, thus providing a mechanism for incorporatingantispoofing methods in the sensor based on determinations of theliveness of the sample.

Similarly, in an embodiment where the sample is a fingertip, themultispectral image pixels are classified as “ridge,” “valley,” or“other,” based on their spectral qualities. This classification can beperformed using discriminant analysis methods such as lineardiscriminant analysis, quadratic discriminant analysis, principlecomponent analysis, neural networks, and others known to those of skillin the art. Since ridge and valley pixels are contiguous on a typicalfingertip, in some instances multispectral data from the localneighborhood around the image pixel of interest are used to classify theimage pixel. In this way, a conventional fingerprint image is extractedfrom the sensor for further processing and biometric assessment. The“other” category may indicate image pixels that have spectral qualitiesthat are different than anticipated in a genuine sample. A threshold onthe total number of pixels in an image classified as “other” may be set.If this threshold is exceeded, the sample may be determined to benon-genuine and appropriate indications made and actions taken.

Biometric determinations of identity may be made using the entiredatacube or particular portions thereof. For example, appropriatespatial filters may be applied to separate out the lower spatialfrequency information that is typically representative of deeperspectrally active structures in the tissue. The fingerprint data may beextracted using similar spatial frequency separation and/or the pixelclassification methods disclosed above. The spectral information can beseparated from the active portion of the image in the manner discussedabove. These three portions of the datacube may then be processed andcompared to the corresponding enrollment data using methods known to onefamiliar with the art to determine the degree of match. Based upon thestrength of match of these characteristics, a decision can be maderegarding the match of the sample with the enrolled data.

As previously noted, certain substances that may be present in the skinand underlying tissue have distinct absorbance characteristics. Forexample, ethanol has characteristic absorbance peaks at approximately2.26 μm, 2.30 μm, and 2.35 μm, and spectral troughs at 2.23 μm, 2.28 μm,2.32 μm, and 2.38 μm. In some embodiments, noninvasive opticalmeasurements are performed at wavelengths in the range of 2.1-2.5 μm,more particularly in the range of 2.2-2.4 μm. In an embodiment thatincludes at least one of the peak wavelengths and one of the troughwavelengths, the resulting spectral data are analyzed using multivariatetechniques such as partial least squares, principal-componentregression, and others known to those of skill in the art, to provide anestimate of the concentration of alcohol in the tissue, as well as toprovide a biometric signature of the person being tested. While acorrelation to blood-alcohol level may be made with values determinedfor a subset of these wavelengths, it is preferable to test at least thethree spectral peak values, with more accurate results being obtainedwhen the seven spectral peak and trough values are measured.

In other embodiments, noninvasive optical measurements are performed atwavelengths in the range of 1.5-1.9 μm, more particularly in the rangeof 1.6-1.8 μm. In specific embodiments, optical measurements areperformed at one or more wavelengths of approximately 1.67 μm, 1.69 μm,1.71 μm, 1.73 μm, 1.74 μm 1.76 μm and 1.78 μm. The presence of alcoholis characterized at these wavelengths by spectral peaks at 1.69 μm, 1.73μm, and 1.76 μm and by spectral troughs at 1.67 μm, 1.71 μm, 1.74 μm,and 1.78 μm. Similar to the 2.1-2.5 μm wavelength range, theconcentration of alcohol is characterized by relative strengths of oneor more of the spectral peak and trough values. Also, while acorrelation to blood-alcohol level may be made with values determinedfor a subset of these wavelengths in the 1.5-1.9 μm range, it ispreferable to test at least the three spectral peak values, with moreaccurate results being obtained when the seven spectral peak and troughvalues are measured.

A small spectral alcohol-monitoring device may be embedded in a varietyof systems and applications in certain embodiments. The spectralalcohol-monitoring device can be configured as a dedicated system suchas may be provided to law-enforcement personnel, or may be integrated aspart of an electronic device such as an electronic fob, wristwatch,cellular telephone, PDA, or any other electronic device, for anindividual's personal use. Such devices may include mechanisms forindicating to an individual whether his blood-alcohol level is withindefined limits. For instance, the device may include red and green LEDs,with electronics in the device illuminating the green LED if theindividual's blood-alcohol level is within defined limits andilluminating the red LED if it is not. In one embodiment, thealcohol-monitoring device may be included in a motor vehicle, typicallypositioned so that an individual may conveniently place tissue, such asa fingertip, on the device. While in some instances, the device mayfunction only as an informational guide indicating acceptability todrive, in other instances ignition of the motor vehicle mayaffirmatively depend on there being a determination that the individualhas a blood-alcohol level less than a prescribed level.

This type of action is an example of a more general set of actions thatmay be performed with the alcohol-monitoring devices of the invention.Such general methods as they may be implemented by thealcohol-monitoring device are summarized in FIG. 9A. At block 902, analcohol-level determination is performed with spectral information asdescribed above. At block 904, a determination is made from thealcohol-level determination whether the alcohol level is withinprescribed limits. If it conforms with such limits, a first action istaken at block 906. This action may correspond, for example, to allowingignition of a motor vehicle, allowing a pilot to enter an aircraft,allowing an employee to enter a workplace, and the like. If thealcohol-level determination does not conform to the prescribed limits, asecond action is taken at block 908. This action may correspond, forexample, to preventing ignition of a motor vehicle, prohibiting accessby a pilot to an aircraft or an employee to a workplace, and the like.

In some instances, the blood-alcohol determination may be coupled with abiometric determination. An overview of such combined methods isprovided with the flow diagram of FIG. 9B. At block 910, analcohol-level determination is performed using spectral information asdescribed above. Different actions may be taken depending on whether thedetermined alcohol level is within prescribed limits, as tested at block912. If the alcohol limit is outside the prescribed limits, a firstaction may be taken at block 914, such as prohibiting ignition of amotor vehicle. Access to the motor vehicle might, however, notautomatically be granted by the system merely because the alcohol levelwas within the prescribed limits. As indicated at block 916, adetermination that those limits are met may instead prompt a biometrictest to be performed so that a check of an individual's identity isperformed at block 918. If the person is identified as a specificperson, such as the owner of the motor vehicle, a second action allowingaccess to the motor vehicle may be taken at block 920. If the personidentified is not the specific person, a third action may be taken atblock 922. This third action could correspond, for example, to the firstaction so that access to the motor vehicle is restricted, but couldalternatively correspond to an action different from the first or secondactions. For example, the third action could result in the sounding ofan alarm to indicate that an unknown person is attempting to gaincontrol of a motor vehicle.

The flow diagrams in FIG. 9B provide examples where a biometric test maybe used to override a decision that would be made in response to aparticular result of an alcohol-monitoring test. In other embodiments, abiometric test could be performed in response to the contrary result forthe alcohol-monitoring test, or could be performed irrespective of theresult of the alcohol-monitoring test. In such cases, different actionscould be taken depending on the various combinations of results of thealcohol-level and biometric determinations. Furthermore, there is noneed for the alcohol-monitoring test to precede the biometricdetermination; the tests could be performed in a different order orsimultaneously in different embodiments.

In some embodiments, correction is made for optical drift by determiningan optical correction from use of the alcohol-monitoring device on areference sample. An overview of a method for making such a correctionis provided in FIG. 9C. At block 932, optical sources of thealcohol-monitoring device are used to illuminate the reference sample,which could conveniently comprise an alcohol-water mixture. At block934, a detector of the alcohol-monitoring device is used to measurespectral characteristics of light after propagation through thereference sample. These spectral characteristics are usually stored forlater application to a variety of different spectral determinations.Thus, at block 936, the light sources of the alcohol-monitoring deviceare used to illuminate tissue of an individual and at block 938, thespectral characteristics of light propagated through the tissue aremeasured with a detector of the alcohol-monitoring device. Before makinga determination of blood-alcohol level using the peak-trough comparisonanalysis described above, the spectral characteristics are corrected inaccordance with the spectral characteristics determined from thereference sample at block 940. Changes that occur to the light sources,detectors, optical filters, lenses, mirrors, and other components in theoptical chain will affect both the in vivo measurement and the referencesample in a similar manner. Processing of the in vivo sample inconjunction with the alcohol-bearing reference sample thus compensatesfor such optical effects.

Management of the functionality discussed herein for thealcohol-monitoring device may be performed with a computer system. Thearrangement shown in FIG. 10 includes a number of components that may beappropriate for a larger system; smaller systems that are integratedwith portable devices may use fewer of the components. FIG. 10 broadlyillustrates how individual system elements may be implemented in aseparated or more integrated manner. The computational device 1000 isshown comprised of hardware elements that are electrically coupled viabus 1026, which is also coupled with the alcohol-monitoring device 1055.The hardware elements include a processor 1002, an input device 1004, anoutput device 1006, a storage device 1008, a computer-readable storagemedia reader 1010 a, a communications system 1014, a processingacceleration unit 1016 such as a DSP or special-purpose processor, and amemory 1018. The computer-readable storage media reader 1010 a isfurther connected to a computer-readable storage medium 1010 b, thecombination comprehensively representing remote, local, fixed, and/orremovable storage devices plus storage media for temporarily and/or morepermanently containing computer-readable information. The communicationssystem 1014 may comprise a wired, wireless, modem, and/or other type ofinterfacing connection and permits data to be exchanged with externaldevices.

The computational device 1000 also comprises software elements, shown asbeing currently located within working memory 1020, including anoperating system 1024 and other code 1022, such as a program designed toimplement methods of the invention. It will be apparent to those skilledin the art that substantial variations may be used in accordance withspecific requirements. For example, customized hardware might also beused and/or particular elements might be implemented in hardware,software (including portable software, such as applets), or both.Further, connection to other computing devices such as networkinput/output devices may be employed

FIG. 10 also indicates that a biometric sensor 1056 may also be coupledelectrically via bus 1026 for use in those embodiments that combine theuse of the alcohol-monitoring device 1055 with the biometric sensor1056. As previously mentioned, the biometric sensor 1056 may also usespectral information in making identifications of individuals, althoughthis is not required. The computational device 1000 may equally well beadapted to coordinate the function of any other type of biometricidentification device with the alcohol-monitoring device as describedabove.

Other analytes in the body may be estimated using similar techniques byensuring that the multispectral data that are measured by the sensorinclude characteristic absorbance features of the analyte of interest.Such analyte estimation techniques may be further aided using a methodsimilar to the pixel classification technique described above. In suchembodiments, the multispectral image pixels are classified as “ridge” or“valley,” or are classified according to another appropriateclassification such as “blood vessel” or “no vessel.” A subset of themultispectral data is the extracted and used for the analyte estimationbased on the pixel classification. This procedure reduces thevariability of the estimation due to optical and physiologicaldifferences across the image plane.

Furthermore, the structural configurations for the sensors describedherein may vary to reflect consideration of such facts as the cost andavailability of off-the-shelf components, materials, designs, and otherissues. Certain configurations may be easier, less expensive, andquicker to build than others, and there may be different considerationsthat affect prototype and volume productions differently. For allembodiments, the optical geometry should be carefully considered. Theregion of skin that returns a detectable amount of diffusely reflectedlight varies considerably as a function of the illumination wavelength.For instance, for visible and very near infrared illumination, theshort-wavelength illumination points may be laid out on a denser arraythan the long-wavelength points. It may be preferable for theembodiments that use swipe configurations to have the timing of theillumination and the image acquisition be sufficient for a relativelyquick motion across the optically active region of the sensor. Amodulated illumination method may advantageously be used for these typesof sensors.

Thus, having described several embodiments, it will be recognized bythose of skill in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the invention. Accordingly, the above description should notbe taken as limiting the scope of the invention, which is defined in thefollowing claims.

1. A sensor system comprising: an illumination subsystem disposed toprovide illumination light to a skin site of an individual; a detectionsubsystem disposed to receive light scattered from the skin site; and acomputational unit interfaced with the detection subsystem and having:instructions for deriving a plurality of spatially distributedmultispectral images from the received light, the plurality of spatiallydistributed multispectral images corresponding to different volumes ofilluminated tissue of the individual; and instructions for identifyingthe individual from the derived plurality of multispectral images. 2.The sensor system recited in claim 1 wherein the instructions foridentifying the individual comprise instructions for searching adatabase of images for an entry identifying an image consistent with thederived plurality of multispectral images.
 3. The sensor system recitedin claim 1 wherein the instructions for identifying the individualcomprise instructions for comparing the derived plurality ofmultispectral images with an image at an entry of a database of imagescorresponding to a purported identity of the individual to verify thepurported identity.
 4. The sensor system recited in claim 1 wherein theillumination subsystem comprises: a light source that provides theillumination light; and illumination optics to direct the illuminationlight to the skin site.
 5. The sensor system recited in claim 4 whereinthe illumination subsystem further comprises a scanner mechanism to scanthe illumination light in a specified pattern.
 6. The sensor systemrecited in claim 4 wherein the light source comprises a plurality ofquasimonochromatic light sources.
 7. The sensor system recited in claim4 wherein the illumination subsystem comprises a broadband light source.8. The sensor system recited in claim 4 wherein the detection subsystemcomprises: a light detector; and detection optics to direct the receivedlight to the light detector.
 9. The sensor system recited in claim 8wherein: the illumination subsystem further comprises a first polarizerdisposed to encounter light emitted from the light source; and thedetection subsystem further comprises a second polarizer disposed toencounter the received light.
 10. The sensor system recited in claim 9wherein the first and second polarizers comprise circular polarizers.11. The sensor system recited in claim 9 wherein the first and secondpolarizers comprise linear polarizers.
 12. The sensor system recited inclaim 11 wherein the first and second polarizers are substantiallycrossed relative to each other.
 13. The sensor system recited in claim 1further comprising a platen disposed to contact the skin site.
 14. Thesensor system recited in claim 13 wherein the platen is adapted for theskin site to be swiped over a surface of the platen.
 15. The sensorsystem recited in claim 14 further comprising a roller system over whichthe skin site may be rolled, wherein the instructions for deriving theplurality of spatially distributed multispectral images includeinstructions for building up the plurality of multispectral images fromlight received from different portions of the skin site as the skin siteis rolled.
 16. The sensor system recited in claim 1 wherein thecomputational unit further has: instructions for derivingspectral-distribution characteristics from the received light; andinstructions for comparing the derived spectral-distributioncharacteristics with a database of spectral-distributioncharacteristics.
 17. The sensor system recited in claim 16 wherein thecomputational unit further has instructions for determining an analyteconcentration in tissue below a surface of the skin site from thederived spectral-distribution characteristics.
 18. The sensor systemrecited in claim 16 wherein the computational unit further hasinstructions for determining a liveness state of the tissue from thederived multispectral characteristics.
 19. The sensor system recited inclaim 1 wherein: the skin site comprises a volar surface of a fingertipor hand; and the instructions for identifying the individual from thederived plurality of multispectral images comprise instructions forderiving a fingerprint of the volar surface from the derived pluralityof multispectral images.
 20. A method for identifying an individual, themethod comprising: illuminating a skin site of the individual withillumination light; receiving light scattered from the skin site;deriving a plurality of spatially distributed multispectral image fromthe received light, the plurality of spatially distributed multispectralimages corresponding to different volumes of illuminated tissue of theindividual; and identifying the individual from the derived plurality ofmultispectral images.
 21. The method recited in claim 20 whereinidentifying the individual comprises searching a database of images foran entry identifying an image consistent with the derived plurality ofmultispectral images.
 22. The method recited in claim 20 whereinidentifying the individual comprises comparing the derived plurality ofmultispectral images with an image at an entry of a database ofmultispectral images corresponding to a purported identity of theindividual to verify the purported identity.
 23. The method recited inclaim 20 wherein illuminating the skin site of the individual withillumination light comprises scanning the illumination light in aspecified pattern.
 24. The method recited in claim 20 whereinilluminating the skin site of the individual with illumination lightcomprises generating the illumination light as a plurality ofquasimonochromatic beams.
 25. The method recited in claim 20 whereinilluminating the skin site of the individual with illumination lightcomprises generating a broadband beam of light.
 26. The method recitedin claim 20 wherein: illuminating the skin site further comprisespolarizing the illumination light with a first polarization; andreceiving the light comprises polarizing the received light with asecond polarization.
 27. The method recited in claim 26 wherein thefirst and second polarizations are substantially crossed relative toeach other.
 28. The method recited in claim 20 further comprising:deriving spectral-distribution characteristics from the received light;and determining an analyte concentration in tissue below a surface ofthe skin site from the derived spectral-distribution characteristics.29. The method recited in claim 20 further comprising: derivingspectral-distribution characteristics from the received light; anddetermining a liveness state from the derived multispectralcharacteristics.
 30. The method recited in claim 20 further comprising:deriving spectral-distribution characteristics from the received light;and comparing the derived spectral-distribution characteristics with adatabase of spectral-distribution characteristics.
 31. The sensor systemrecited in claim 20 wherein: the skin site comprises a volar surface ofa fingertip or hand; and identifying the individual from the derivedplurality of multispectral images comprises deriving a fingerprint ofthe volar surface from the derived plurality of multispectral images.32. A sensor system comprising: an illumination subsystem disposed toprovide light at a plurality of discrete wavelengths to a skin site ofan individual; a detection subsystem disposed to receive light scatteredfrom the skin site; and a computational unit interfaced with thedetection subsystem and having: instructions for deriving a plurality ofspatially distributed multispectral images from the received light atthe plurality of discrete wavelengths, the plurality of spatiallydistributed multispectral images corresponding to different volumes ofilluminated tissue of the individual; and searching a database of imagesfor an entry identifying an image consistent with the derived pluralityof multispectral images.
 33. The sensor system recited in claim 32wherein: the database of images includes multispectral images.
 34. Thesensor system recited in claim 32 wherein: the database of imagesincludes conventional fingerprint images.